Ultrasonic bonding has many applications in the electronics industry. For example, it is common to attach solid-state components such as transistors or integrated circuits to a substrate or package. Frequently, the terminals of the components are then interconnected to each other or the conductor circuits of the substrate by small diameter wires that are attached by ultrasonic bonding. As is well known in the art, ultrasonic bonding may be performed by pressing the bonding tool or wedge against the wire contacting the terminal and then vibrating the tool at an ultrasonic frequency, such as, for example, 60 KHz. Various ultrasonic bonding machines having precise control over bonding parameters such as power, duration, downward force, and work piece positioning are commercially available and in wide usage.
A substantial semiconductor industry problem with ultrasonic bonding has been evaluating the quality of individual bonds. By providing an evaluation of the bond quality, the operator may alter operation of bond settings to get a higher quality yield. Also, the bonds of lower quality can be weeded out or discarded to provide a product having much higher predictable reliability. In many systems and particularly in military applications, reliability is of the utmost importance.
One prior art ultrasonic bonding evaluation method is to measure the required force to pull apart a certain percentage of the bonds. Although this method provides some data from which a prediction can be made regarding the reliability of the bonds not destroyed, it obviously does not sort the faulty bonds to increase reliability. Furthermore, this method is very time consuming, wasteful in that bonds are destroyed, and somewhat unreliable unless the destructive percentage is large.
Another prior art evaluation method for predicting bond reliability involves nondestructive pull testing in which the bonds are mechanically stressed to a preestablished level below the level that a good bond would pull apart. Other than being time consuming, this method provides no bond quality data about the force actually required to pull bonds apart. Also, this method damages some bonds that would otherwise be acceptable.
Another method which is described in U.S. Pat. No. 3,827,619 to Cusick et al, issued Aug. 6, 1974, utilizes a voltage which is proportional to the amplitude of the traverse motion of the ultrasonic bonding tool and a second voltage proportional to the tangential component of the force applied during bonding. In short, this method is based on the proposition that bond quality is proportional to the forces in the X and Y direction applied during the bonding process. Although this method may have been advantageous over the nondestructive pull type and destructive methods, there is a requirement to more accurately determine the actual quality of individual ultrasonic bonds.